Influenza A virus (IAV) targets lung epithelial cells and exploits the host cell machinery to replicate, causing respiratory illness in annual epidemics and pandemics of variable severity. Influenza affects all age groups, results in considerable morbidity and mortality, and exacts a formidable toll on world health and economics. Seasonal influenza causes 250,000 to 500,000 deaths worldwide and about 36,000 deaths in United States annually. Current therapies target the virus and are therefore limited by the capacity of influenza to rapidly evolve and are frequently ineffective in critically ill patients. One critical aspect of IAV-induced pathogenesis is lung epithelial cell death and inflammation, which compromises pulmonary function and contributes to disease morbidity and mortality. Hence it has been proposed that identification of and targeting key inducible host cell factors that regulate IAV-induced lung injury, inflammation and viral replication may provide a potential solution to combat future influenza outbreaks. We recently reported that transcription factor Runx3, but not Runx1 and Runx2, was induced by IAV infection, viral RNA, a synthetic double-stranded RNA (dsRNA) analog polyinosinicpolycytidylic acid (poly(I:C)) and interferon-? (IFN?) in normal human airway epithelial cells and demonstrated that Runx3 played a crucial role in the cell death such as apoptosis induced by IAV infection and dsRNA. Moreover, our new preliminary data indicate that Runx3 can be induced in IAV-infected mouse airway and alveolar epithelial cells in vivo and is an important mediator of the expression of dsRNA receptor Toll-like receptor 3 (TLR3) that contributes to a detrimental host inflammatory response to IAV infection. As IAV initially infects the airway epithelium and induces robust innate immune response, our novel findings prompt us to develop mice with airway epithelium specific knockout (KO) of Runx3 to test the hypothesis that the inducible transcription factor Runx3 may be a novel and crucial regulator of airway epithelial cell pathogenic responses to IAV infection and the disease progression of influenza in vivo. Two specific aims will be tested in this proposal. Aim-1 will generate airway epithelium specific Runx3 conditional KO mice and determine the effect of Runx3 conditional KO on lAV-induced disease progression; Aim-2 will determine the role and mechanism of Runx3 in IAV-induced lung injury and inflammation. This R21 proposal will reveal the airway epithelium specific role of Runx3 in host responses to IAV infection and the disease progression of influenza. This proposal will also enable us to assess if controlling the initial pathogenic response in the airway epithelial compartment could limit the overall lung injury and inflammation, which may lead to recognition that modulation of Runx3 offers a novel strategy to control host pathogenic response to the infection of virulent IAV.